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The Neoproterozoic and Cambrian of the Tethyan Himalaya: a Test of Models of Core Gondwanan Construction

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The Neoproterozoic and Cambrian of the Tethyan Himalaya: a Test of Models of Core Gondwanan Construction THE NEOPROTEROZOIC AND CAMBRIAN OF THE TETHYAN HIMALAYA: A TEST OF MODELS OF CORE GONDWANAN CONSTRUCTION Nigel Hughes and Paul Myrow, Principal Investigators PROJECT SUMMARY: Tectonic reorganization, including breakup of the supercontinent Rodinia and the subsequent assembly of Gondwanaland, is part of the remarkable changes to the Earth, atmosphere, and biosphere that occured during the Neoproterozoic to Cambrian interval. The nature and timing of the assembly of core Gondwanaland remains a matter of vigorous debate. The more traditional view of Gondwanan assembly is that East Gondwana, consisting of India, Australia and Antarctica collided with the combined African cratons (Kalahari and Congo) as one block sometime during the late Neoproterozoic. Estimates of the timing of this collision are range between 700 to 600 Ma. An alternative model suggests that the accretion of East Gondwana to the African craton was a two part process. In this model, India broke off from Australia and Antarctica, swept across the Mozambique Ocean, and collided with the African craton at approximately 680 Ma. This was purportedly followed sometime later by collision of Australia/Antarctica around 550 to 530 Ma. Our proposed study of the Tethyan Himalaya will test tectonic models of the assembly of core Gondwanaland using a combined sedimentological, macrofaunal and stratigraphic approach. The Tethyan Himalaya was in a critical paleogeographic position to capture a stratigraphic signal of one or more of these proposed tectonic events. It also contains the most complete succession of Neoproterozoic through Cambrian deposits in India. Several predictions should be able to distinguish the two models of core Gondwanan assembly. Of primary importance is the stratigraphic signature of sediment input from adjacent orogenic belts. A record of major increases of siliclastic sediment, with abrupt changes in paleocurrents would accompany orogenesis. Such changes are readily recognizable in the stratigraphic record. A key component of the proposed models is that they have predictable patterns of detrital zircon ages. The collision with the western margin of India would result in deposition of sediment sourced from the pan-African Mozambique Fold and Thrust Belt, with detrital ages generally between 600-700 Ma. The proposed collision along eastern India in the proposed two-stage model would have resulted in deposition in the Tethyan Himalaya of sediment sourced from southwest Australia, which would have been a source of detrital zircons with a wide variety of Precambrian ages including those >3.0 Ga. A Late Cambrian–Early Ordovician tectonic event is recorded in the Tethyan Himalayas and in other places in East Gondwanaland. The nature of this event is unclear, but two possibilities include: (1) it represents a somewhat delayed expression of the Australia/Antarctica collision in the two-stage model, or (2) it represents the docking of an outboard microcontinent (e.g., the Lhasa Block of Tibet) subsequent to the formation of core Gondwanaland. The biostratigraphic patterns of trilobites provide the basis of testable predictions of these hypotheses. In the first case, we expect to find a stratigraphic increase in similarity in faunas between India and the rest of East Antarctica throughout the Cambrian. If faunal similarity with the rest of East Gondwana was constant throughout the trilobite-bearing Cambrian, we may infer that Gondwanan construction was complete prior to 520 Ma. In addition, if the ~500 Ma tectonic event in the Tethyan Himalaya records microcontinental collision, then the identification of the outboard terrane may be approached through biogeographic patterns established in this study. As a compliment to detailed sedimentological, macrofaunal and stratigraphic analyses, we will also conduct exploratory carbon isotope stratigraphy of carbonate units, microfossil analysis, and geochronology of volcanic ash samples. Myrow’s RUI proposal will involve undergraduates at Colorado College as junior collaborators in all components of the research. This will provide a much-needed opportunity for students to experience scientific research in diverse geographical and cultural environments. A key aspect of this proposal is the close research collaboration with Indian and Chinese scientists. INTRODUCTION AND STATEMENT OF THE PROBLEM The Neoproterozoic through Cambrian interval was a time of radical reorganization of tectonic plates with the fragmentation of the supercontinent Rodinia and subsequent assembly of “core” Gondwanaland (comprising South America, Africa, Madagascar, Greater India, Australia and Antarctica). Numerous tectonic reconstructions have been proposed for this reorganization, based in part on emerging paleomagnetic data, which include apparent polar wander paths. It is generally agreed that Rodinia was assembled by 1050 Ma with Laurentia at its core (Bond et al. 1984; Dalziel 1992; Weil et al. 1998; Hoffman 1999). Continental blocks broke off of Rodinia sequentially and moved in a fan-like rotation from both the eastern and western sides of Laurentia towards the African Congo and Kalahari cratons, which had merged by approximately 820 Ma (Hanson et al. 1993; Hoffman 1999). Those fragments that would later make up East Gondwana broke off from western Laurentia and swept across the Mozambique Ocean to collide with the African cratons to form the East African Orogen (Stern 1994). First-order problems remain concerning the nature and timing of accretion of Greater India/Madagascar and Australia/Antarctica with the eastern margin of Africa. The presence of several “outboard” terranes that jostled about the peri-Gondwanan margin during the early Paleozoic (Metcalfe 1993, 1996) further complicates the picture. This project will focus on latest Terminal Proterozoic and Cambrian rocks in the Tethyan Himalaya of northern India and Tibet as a test of two competing hypotheses for the closure of the Mozambique Ocean and the final stages of core Gondwanan assembly. The more traditional view is that the core of Gondwanaland was assembled when the East Gondwanan blocks of India, Australia, and Antarctica, moving as one, docked against the Congo and Kalahari cratons (Dalziel 1992; Li and Powell 1993). The dates given for the collision of East and West Gondwanaland according to this model vary from 700 Ma (Powell, et al. 1993, Stern 1994) to about 510 Ma (Kaz’min 1988; Li et al. 1993, 1996; Powell, et al. 1993; Trompette 1994). A recent model by Meert and Van der Voo (1997), which is based mainly on paleomagnetic data, reconciles some of these age disparities by proposing that the construction of eastern Gondwanaland was a two-phase process with the initial collision of Greater India with East Africa occurring some 680 Ma ago, followed by the subsequent collision of Australia/Antarctica with India around 550-530 Ma. The Tethyan Himalaya was in a critical position to record the two proposed stages of East Gondwanan assembly (Meert and Van der Voo 1997) because in tectonic reconstructions the wedge-shaped Indian block forms a juncture with Antarctica (Darling Orogen) and the Congo/Kalahari Craton (Mozambique Belt). Its inferred position places it in proximity to both sutures and so the effects of each proposed tectonic event should be recorded in the sedimentary succession, in part as a two-stage influx of sediment, that records a shift in sediment source provenance and paleocurrents. It is the only region on the Indian subcontinent which preserves a succession of sedimentary rocks with an extensive record of Neoproterozoic through Middle Cambrian rocks. This study will test the competing hypotheses of core Gondwanan assembly with a thorough field-based analysis of stratigraphic sections along the strike of the Himalaya. It will be a major step towards an integrated model of the regional paleogeography and margin development during the latest Terminal Proterozoic through Cambrian. Within the Tethyan Himalaya there is clear evidence of a major Late Cambrian–Early Ordovician orogenic event. However, the significance of this ~ 500 Ma event is poorly understood. It is unclear whether it is related to the construction of core Gondwana or whether this represents a separate event, such as the collision of an as-yet unidentified outboard continental block with the Tethyan margin. The project will be an integrated stratigraphic, paleontologic, and sedimentologic analysis. It will provide badly needed chronostratigraphic constraint for a section for which there is a meager biostratigraphic database and almost no information on depositional history, paleoenvironments, or large- scale stratigraphic architecture (including sequence stratigraphic frameworks). The project will specifically focus on sections in Kashmir and Spiti/Zanskar in India, and Nyalam in Tibet. The region is remote and difficult to access so many potentially important outcrops have not yet been analyzed with integrated, modern, stratigraphic approaches. A key aspect of this study is that it will be conducted in close collaboration with geologists from India (Dr. S.K. Parcha, Wadia Institute of Himalayan Geology) and China (Dr. Shanchi Peng, Academia Sinica, Nanjing), both of whom have extensive field experience in the field areas. Drs. Parcha and Peng will be active co-authors in the resultant research, and their involvement will also greatly facilitate implementation of this field-based research program. In addition to their skills as scientists, their institutions have pledged logistic support (Appendix).
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